The Role of Neutrophils in the Induction of Glomerulonephritis by Anti-Myeloperoxidase Antibodies (1)
Anti-neutrophil cytoplasm autoantibodies (ANCAs) crescentic glomerulonephritis (NCGN) is the most common form of aggressive glomerulonephritis and often accompanied by a pauci-immune systemic necrotizing small vessel vasculitis, such as microscopic polyangiitis or Wegener’s granulomatosis. In vitro, ANCAs can activate cytokine-primed neutrophils, causing an oxidative burst, degranulation, release of inflammatory cytokines, and damage to endothelial cells.
Studies of the effects of neutrophil depletion on induction of glomerulonephritis used one intravenous injection of 50 ug/g body weight of anti- MPO IgG in phosphate-buffered saline (PBS) on day 0 with sacrifice on day 6. The first intravenous 50 ug/g body weight dose of anti-MPO IgG was administered on day 0, the second dose was administered on day 3, and the mice were sacrificed on day 6.
Within 1 day after a single injection of 1 mg of NIMP-R14 monoclonal antibody in 0.5 ml of PBS into B6 mice (n 7), the number of circulating neutrophils was dramatically reduced from 14% of white blood cells to 1%, and remained at this low levels for up to 5 days.
After 5 days, mice injected with anti-MPO IgG without neutrophil depletion developed urine abnormalities consistent with glomerulonephritis, ie, 2.5 +/- 0.5 hematuria, 1.9 +/- 0.55 proteinuria, and 0.8 +/- 0.8 leukocyturia. These differences were reduced by the depletion of circulating neutrophils.
Neutrophils were most numerous in glomeruli with inflammation and necrosis, and macrophages tended to cluster within crescents.
Robusted glomerular injury was established with two doses of anti-MPO antibodies given at day 0 and day 3. These mice developed hematuria (2.6+), proteinuria (2.8+), leukocyturia (1.4+), and elevated blood urea nitrogen (45.2 mg/ dl). Even in the mice that received two doses of anti-MPO, most glomeruli had no abnormalities by light microscopy.
Immunohistological staining of paraffin sections with NIMP-R14 for neutrophils demonstrated that induction of glomerulonephritis by anti-MPO IgG was accompanied by glomerular influx of neutrophils.
Numerous in vitro studies have documented the ability of human ANCA IgG (ie, both anti-MPO and anti-PR3 IgG) to activate neutrophils with the resultant release of toxic oxygen metabolites, lytic and toxic proteases, nitric oxide, and inflammatory cytokines.
This neutrophil activation involves increased display of ANCA antigens at the surface of neutrophils facilitating both the attachment of the antigen-binding portion of the autoantibodies as well as the engagement of Fc receptors on the surface of neutrophils. Monocytes express both MPO and PR3 and can be activated by ANCA IgG to release many of the same mediators that are released by neutrophils after activation by ANCA.